git_ui: Design Polish (#26361)

Polish PR

- [ ] Horizontal scrollbar for git panel
- [ ] Allow shift clicking a checkbox in any section to stage the whole
section
- [ ] Clean up design of no changes/pending push state in panel
- [x] Ensure checkbox placeholder dot is centered in the checkbox
- [x] Improve spacing between elements in git panel entries
- [x] Update git branch icon to match branch selector text when disabled
- [x] Truncate last commit message less aggressively in panel
- [x] Clean up new panel header design
- [x] Remove `_background` version control keys (backgrounds are derived
from the foreground colors)

### Previous message truncation:

Before:

![CleanShot 2025-03-10 at 11 54
32@2x](https://github.com/user-attachments/assets/46b18f66-bb5c-435e-a0da-6cc931bd8a15)

After:

![CleanShot 2025-03-10 at 11 55
24@2x](https://github.com/user-attachments/assets/fcf688c7-b949-41a2-a7b8-1a198eb7fa4a)

### Make branch icon match when menu is disabled

Before:

![CleanShot 2025-03-10 at 12 02
14@2x](https://github.com/user-attachments/assets/1990f4b3-c2f0-4e02-89ad-211aaebb3821)

After:

![CleanShot 2025-03-10 at 12 02
53@2x](https://github.com/user-attachments/assets/9b1caf65-c48f-44c9-924b-484892fb543f)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

---------

Co-authored-by: Cole Miller <cole@zed.dev>
Co-authored-by: Cole Miller <m@cole-miller.net>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/panel/src/panel.rs

@@ -18,8 +18,6 @@ pub trait PanelHeader: workspace::Panel {
             .w_full()
             .px_1()
             .flex_none()
-            .border_b_1()
-            .border_color(cx.theme().colors().border)
     }
 }